The UMTS (universal mobile telecommunications system) is a third generation mobile communications system evolved from the European GSM (Global System for Mobile Communications) system and aims to provide improved mobile communication services based on a GSM core network and a wideband code division multiple access (W-CDMA) technology.
FIG. 1 is a block diagram illustrating a structure of a UMTS network.
Referring to FIG. 1, the UMTS system generally consists of a terminal (e.g., user equipment (UE)), a UTRAN (UTMS radio access network) and a CN (core network). The UTRAN includes one or more RNSs (radio network sub-systems). Each RNS includes a single RNC (radio network controller) and one or more base stations (e.g., node-Bs) managed by the RNC. Each Node B managed by the RNC receives information sent from a physical layer of a terminal on the uplink, and transmits data to the terminal on the downlink. Thus, the node-B serves as an access point to the UTRAN for the terminal. The RNC allocates and manages radio resources, and serves as an access point to the CN for the UTRAN.
FIG. 2 is a diagram illustrating a connection structure between a UTRAN and a terminal in a UMTS network. As such, FIG. 2 shows an exemplary architecture of radio interface protocols between a terminal and a UTRAN based on a 3GPP radio access network specification.
Referring to FIG. 2, the radio protocol is horizontally arranged to include a physical layer, a data link layer and a network layer. The radio protocol is vertically divided into a user plane for transmitting data information and a control plane for transferring control signals. Furthermore, the protocol layers shown in FIG. 2 may be divided into a first layer L1, a second layer L2 and a third layer L3, based upon the three lower layers of an open system interconnection (OSI) specification model that is well-known in the art of communications systems.
The first layer L1 (e.g., the physical layer) uses physical channels to provide information transfer service to its upper layers. The physical layer is connected via a transport channel to a medium access control (MAC) layer located at an upper layer thereof. Data between the medium access control layer and the physical layer is transferred via this transport channel. Furthermore, the data between different physical layers, i.e. between physical layers of transmitting side and receiving side, is transferred via the physical layer.
The MAC layer of the second layer L2 provides services to an upper radio link control (RLC) layer via a logical channel. The RLC layer of the L2 supports reliable data transmissions and performs segmentation and concatenation of RLC SDUs (service data units).
The RRC (Radio Resource Control) layer positioned in the lowest portion of the third layer (L3) is defined in the control plane and controls the transport channels and the physical channels for establishment, reconfiguration, and release of radio bearers (RBs). The RB indicates a service by the L2 for a data transfer between the terminal and the UTRAN. In general, establishing the RB denotes processes of stipulating the characteristics of a protocol layer and a channel, which are required for providing a specific service, and setting corresponding parameters and operation methods.
When the RRC layer of a specific terminal and the RRC layer of the UTRAN are connected to each other to exchange RRC messages, the corresponding terminal is in a RRC connected state. On the contrary, when not connected to each other, the corresponding terminal is in an idle state. The terminal in the RRC connected state is further divided into URA_PCH state and CELL_PCH state, CELL_FACH state and CELL_DCH state. Terminals in the idle state or the URA_PCH state and CELL_PCH state use a discontinuous reception (DRX) method to reduce power consumption, thereby discontinuously receiving a physical channel (i.e., SCCPCH (secondary common control physical channel)) to which a physical channel (i.e., PICH (paging indicator channel)) and a transport channel (i.e., PCH (paging channel)) are mapped. The terminal is in a sleeping mode during the remaining time periods excluding those times when receiving the PICH or the SCCPCH. The terminal performing the DRX method wakes up once per DRX cycle length to receive a PI (paging indicator) of the PICH channel.
The terminal in the RRC connected state may further have a signaling connection with the CN. In this case, the signaling connection indicates a path for exchanging control messages between the terminal and the CN. The RRC connected state denotes a connection between the terminal the UTRAN, while the signaling connection denotes a connection between the terminal and the CN. The terminal uses the signaling connection to inform the CN of its position and/or to request services. The terminal should be in the RRC connected state to have a signaling connection.
A MBMS (multimedia broadcast/multicast service) is described below.
The MBMS refers to a service for providing streaming or background services to a plurality of terminals using a downlink dedicated MBMS bearer service. In the UTRAN, the MBMS bearer uses a point-to-multipoint radio bearer (RB) and a point-to-point RB. At this time, the point-to-multipoint RB uses a common channel (e.g., using the SCCPCH as a physical layer) so that a plurality of terminals subscribed to a corresponding service may receive the service.
The MBMS is divided into a broadcast mode and a multicast mode. The MBMS broadcast mode is a service for transmitting multimedia data to all users within a broadcast area. Conversely, the MBMS multicast mode is a service for transmitting the multimedia data only to a specific user group within a multicast area. The broadcast area refers to a region in which it is possible to transmit the broadcast service, and the multicast area refers to a region in which it is possible to transmit the multicast service.
FIG. 3 is a diagram illustrating an example of the UMTS network providing a point-to-multipoint service to the terminal.
Referring to FIG. 3, a UE1 performs a subscribing process in order to receive an MBMS service. The UE1 also receives a service announcement provided by the network regarding the MBMS service. Subscribing refers to the establishment of a relationship between a service provider and a user. A service announcement refers to providing the terminal an index and other information related to the services to be provided. For example, a terminal that desires to receive the MBMS service of a multicast mode should join a multicast group. The multicast group refers to a group of terminals receiving a specific multicast service. Joining refers to a user merging with other users in a multicast group to receive a particular multicast service. The terminal may inform the UMTS network that it desires to receive specific multicast data through MBMS multicast joining. On the contrary, leaving is a procedure in which a terminal that joined a specific multicast group releases its joining to the multicast group. Each terminal performs such subscribing, joining and leaving processes. The terminal may perform the subscribing, joining, and leaving processes any time of before, during, or after data transmission.
During performance of a specific MBMS service, one or more sessions may sequentially be generated with respect to the MBMS service. If data to be transmitted for the specific MBMS service is generated in a MBMS service source, the CN notifies a session start to the RNC. However, if there is no data to be transmitted for the specific MBMS service in the MBMS service source, the CN notifies a session end to the RNC. The data transmission with respect to the specific MBMS service may be carried out between the session start and the session end. At this time, the data transmitted through the data transmission process is transmitted only to terminals that have joined the multicast group.
In the session start process, the UTRAN receives the session start notification from the CN and transfers a MBMS notification to the terminals. The MBMS notification refers to the UTRAN informing a terminal that the data transmission of the specific MBMS service in a cell is imminent. The MBMS notification may be transmitted multiple times before the transmission of the data pertaining to the service. When carrying out the MBMS notification process, the UTRAN may also count the number of terminals receiving the specific MBMS service within a cell. The counting may be used for determining whether a point-to-multipoint RB or a point-to-point RB is set as the RB for transmitting the specific MBMS service data, or whether no RB is set.
The UTRAN internally establishes a threshold value to select an appropriate MBMS RB. After the UTRAN counts the number of terminals, if the counted number of terminals receiving the corresponding MBMS service within the corresponding cell is smaller than the threshold value, the UTRAN establishes a point-to-point RB. However, a point-to-multipoint RB is established if the counted number of terminals is greater than the threshold value. After determining the MBMS RB, the UTRAN informs the terminals of the establishment information of the corresponding RB.
When the point-to-point RB is established for the specific service, the terminals that desire to receive the service must be in the RRC connected mode state. However, when the point-to-multipoint RB is established for the specific service, the terminals that desire to receive the service do not have to be in the RRC connected mode state. That is, terminals in an idle state may also receive the MBMS service data by using the point-to-multipoint RB. However, if no terminals are counted, the UTRAN does not establish any RB and does not transmit the MBMS service data. As such, the UTRAN establishing an RB when there is no user who wants to receive the service may cause undesirable consumption of radio resources. The UTRAN initiates a transmission for MBMS service data received from the CN during a session using the determined RB.
During the counting process, the UTRAN does not have any information pertaining to terminals in an RRC idle mode. Accordingly, when the terminals in the RRC idle mode are requested by the UTRAN for counting (with respect to MBMS services subscribed by the terminals), the terminals in the RRC idle mode implement an RRC connection with the UTRAN. As a result, the terminals notify the UTRAN that the terminals in the RRC idle mode want to receive the specific MBMS service.
However, when the terminals have a signaling connection with the SGSN (serving general packet radio service support node), the SGSN informs the UTRAN of information related to the MBMS of the terminals. The information includes a list of MBMS services subscribed to by the terminals. Therefore, the UTRAN may recognize whether the terminals have subscribed to a specific MBMS service. Accordingly, the terminals do not respond to the request for the counting from the UTRAN. Terminals which do not have the signaling connection with the SGSN but are in the RRC connected state may inform the UTRAN of the list of MBMS services subscribed to by the terminals. Hence, the UTRAN may recognize the number of terminals desiring to receive the specific MBMS service without receiving any response from the terminals in the RRC connected state.
The UTRAN may perform the counting process during a session of the MBMS service or in the initial step of the MBMS service. This is because, during the MBMS session, the terminal may move into another cell, turn off its power, or discontinue use of the MBMS service. Therefore, the number of terminals desiring to receive the MBMS service in a cell may change. Thus, the UTRAN may perform the counting process to establish the RB more efficiently even while the MBMS session is ongoing.
A FLC (Frequency Layer Convergence) is described below.
In the UMTS system, a base station may actually use one or more frequency bandwidths. That is, because all users may not receive services with appropriate qualities using only one frequency bandwidth in an area where many users are congregated (e.g., in a hot-spot), service providers consider the service demand in a region where the base station is positioned in order to provide services by using more than one frequency bandwidth.
When one base station uses several frequencies, for example several adjacent frequencies, the propagation characteristics of radio waves in each frequency have many similarities. If one MBMS service is provided using a power level corresponding to approximately ten percent of the entire power available for a specific frequency in a specific cell, when the base station provides the same MBMS service at other frequencies, significant radio resources may be consumed.
Therefore, when a base station uses several frequencies, the base station selects one frequency among the available frequencies to provide the MBMS service. The terminals then move to the selected frequency. The preceding operations are referred to as FLC (Frequency Layer Convergence). The specific frequency selected by the base station to provide the MBMS service is referred to as a preferred frequency.
In FLC, when notifying terminals of a MBMS start, the base station informs the terminals of the preferred frequency on which the MBMS service is provided to induce the terminals to move to the specific frequency. To this end, the base station informs the terminals positioned at frequencies other than the preferred frequency that the MBMS service is provided only at the preferred frequency. Then, the terminals carry out a cell reselection procedure. When a condition for changing a cell is satisfied, the terminals reselect a cell to move to the preferred frequency.
The cell reselection by the terminals is described below.
The cell reselection process is a process for positioning a terminal in a cell that provides services of at least a minimum quality. In this process, the terminal compares the quality of a serving cell against the quality of a neighboring cell. If the quality of the neighboring cell is superior to that of the serving cell, the terminal reestablishes the neighboring cell as its serving cell.
Cell quality may be influenced by radio wave environment, which may change in an irregular manner. As a result, a terminal located at a boundary of more than two cells frequently performs cell reselection in an alternating fashion (e.g., selecting different cells in turn). Whenever cell reselection is performed, the terminal must re-register its location with a new cell or temporarily stop data transmissions. Accordingly, cell reselection should not be performed more frequently than necessary, to prevent waste of resources. Accordingly, when the quality of the serving cell is higher than a specific reference value, the terminal does not perform cell reselection. The specific reference value is set at a value by which services may be provided at a satisfactory level to the terminal. That is, even if the serving cell does not provide better quality than neighboring cells, if the quality of the serving cell is higher than the specific reference, then the terminal does not perform cell reselection.
When a base station uses several frequencies, each frequency may be for a particular cell, and the cell reselection is used in Cell_PCH, Cell_FACH, URA_PCH and idle mode in which the UTRAN does not assign a cell where the terminal should be positioned.
FLC is described in relationship with cell reselection, below. When a frequency in a cell in which a terminal is positioned is not a preferred frequency, the terminal receives information regarding the preferred frequency from a base station at an initial step of the MBMS service. However, as described above, if the quality of the cell in which the terminal is positioned exceeds a specific reference value, the terminal does not perform the cell reselection process. Because the MBMS service subscribed by the terminal is provided only at the preferred frequency, if the terminal does not move to the preferred frequency, the terminal may not receive the service to which it subscribed. Therefore, in order for the terminal to receive the service to which it has subscribed, when a specific MBMS service is provided only at a preferred frequency and the terminal does not position itself at the preferred frequency, the terminal performs the cell reselection process even if the quality of the serving cell where the terminal is positioned is higher than the specific reference value.
In the cell reselection process, the base station informs the terminal of an offset value to facilitate the selection of the preferred frequency by the terminal. The offset value is added to a quality measurement value of a cell positioned at the preferred frequency or is excluded from a quality measurement value of a cell not positioned at the preferred frequency. A specific frequency or a specific cell is determined to have a quality better than that of other cells based on this offset value. Accordingly, the base station may increase the probability that the terminal moves to the preferred frequency or the specific cell.
As mentioned above, the FLC is a method for positioning a plurality of terminals at a specific frequency bandwidth in a certain area. That is, using this FLC method, the base station may reduce consumption of radio resources to provide the MBMS service to the terminals.
However, the FLC deliberately changes a distribution of terminals in order to position many terminals at a preferred frequency. As a result, many terminals are positioned at the preferred frequency, but a relatively small number of terminals are positioned at other frequencies. Therefore, a load is converged only upon a specific cell, causing an imbalance between the frequencies and cells. In addition, as compared to a uniform distribution of terminals over several cells, when FLC is implemented, the terminals may receive services of lesser quality, resulting in an inefficient allocation of radio resources.
As described above, at the initial step of the MBMS service, the terminal performs the cell reselection process to move into the cell of the preferred frequency on the basis of information transmitted from the base station even if the quality of the serving cell in which the terminal is positioned exceeds a certain reference value. However, after completing the MBMS service, when the quality of the cell in which the terminal is positioned exceeds the specific reference value, the terminal does not move into a new cell but remains positioned at the preferred frequency. That is, the state of the MBMS service start, i.e. that terminals positioned at other frequencies are converged upon the preferred frequency, continues until after the MBMS service is completed. A problem may occur, however, in which the number of terminals to which a cell may provide services with the appropriate quality is exceeded. This problem may continue even after the MBMS service has ended.
In general, if the terminal is positioned in a cell with the best quality, much data may be exchanged with the base station even with relatively low power. However, depending on the current FLC method of the MBMS service, although the terminal may receive a service with far better quality from another frequency or cell after the MBMS service is completed, the terminal does not change the frequency. Instead, the terminal remains continuously positioned at the same frequency as long as the quality of the currently selected cell is higher than a certain reference value. This distribution of terminals after the MBMS service is completed may waste power in terminals.